As. Davis et al., CHEMICALLY DIVERSE, SPORADIC VOLCANISM AT SEAMOUNTS OFFSHORE SOUTHERNAND BAJA-CALIFORNIA, Geological Society of America bulletin, 107(5), 1995, pp. 554-570
Compositions of lavas from seven small to medium-sized seamounts, betw
een lat 34.0 degrees N and 30.5 degrees N offshore southern and Baja C
alifornia, include low K2O tholeiitic, transitional, and mildly to mod
erately alkalic basalt and their differentiates. The low-K2O tholeiite
s resemble primitive (>9% MgO) mid-oceanic-ridge basalt (MORE) with lo
w incompatible element abundances and very depleted, concave downward,
chondrite-normalized rare-earth element (REE) patterns and lower Sr-8
7/Sr-86 and higher Nd-143/ Nd-144 ratios than typical MORE fi om the E
ast Pacific Rise. The seamounts with these MORE-like lavas are inferre
d to have formed at or near the spreading center. Transitional and mil
dly to moderately alkalic basalts have higher abundances of incompatib
le elements and steeper slopes for chondrite-normalized REE patterns w
ith light REE enrichment up to 150 times chondrites. The alkalic compo
sitions indicate more variably enriched mantle sources than those of m
ost seamounts presently located near the East Pacific Rise, but the co
mpositions are within the mantle array defined by other ocean-island b
asalts. Volcanic rocks from the upper part of Rocas Alijos, a much lar
ger and morphologically more complex edifice than the northern seamoun
ts, located offshore central Baja California at lat similar to 25 degr
ees N, are all highly differentiated trachyte and trachyandesite. Base
d on Ar-40/Ar-39 laser fusion techniques, MORE-like lava from one of t
he northern edifices is as old as the underlying oceanic crust (>20 Ma
), indicating that it originated at a spreading center. Other seamount
lava ages are much younger than the oceanic crust on which they resid
e, ranging from 16.8 +/- 0.3 to <7 Ma for some of the northern seamoun
ts to 270 +/- 16 ka for the trachyte from Rocas Alijos. Similar highly
evolved lavas cap fossil spreading centers like Guadalupe and Socorro
Islands, but Rocas Alijos, based on magnetic anomalies, is not an aba
ndoned spreading center but may instead have formed on a leaky transfo
rm fault. Some of the seamounts with transitional and alkalic lavas ma
y have formed as part of a short, age-progressive chain formed by a sh
ort-lived mantle plume. Many others, aligned along abandoned spreading
centers or faults and fracture zones which are abundant in the tecton
ically complex region offshore southern and peninsular California, may
have resulted from upwelling mantle diapirs in response to localized
extension. Some of the episodes of volcanism appear to have been conte
mporaneous with volcanism in the continental borderland and coastal so
uthern California, suggesting linkage between extension along the cont
inental margin and the seamount province farther offshore. The data av
ailable for the abundant volcanic edifices of varying sizes, shapes, a
nd orientations in this region suggest that the seamounts formed from
multiple episodes of chemically diverse volcanism, tapping variably en
riched, heterogeneous mantle, which occurred sporadically from early M
iocene to late Pleistocene.